cron: use Lua table rather than native array

Pin a table to the registry and use that instead of manually allocating
and reallocating and shuffling integers around.

Thanks to @TerryE for some style suggestions.

app/modules/cron.c

@@ -27,8 +27,7 @@ typedef struct cronent_ud {
 
 static ETSTimer cron_timer;
 
-static int *cronent_list = 0;
-static size_t cronent_count = 0;
+static int cronent_table_ref;
 
 static uint64_t lcron_parsepart(lua_State *L, char *str, char **end, uint8_t min, uint8_t max) {
   uint64_t res = 0;
@@ -86,12 +85,20 @@ static int lcron_parsedesc(lua_State *L, char *str, struct cronent_desc *desc) {
 static int lcron_create(lua_State *L) {
   // Check arguments
   char *strdesc = (char*)luaL_checkstring(L, 1);
-  void *newlist;
   luaL_checktype(L, 2, LUA_TFUNCTION);
-  // Parse description
-  struct cronent_desc desc;
-  lcron_parsedesc(L, strdesc, &desc);
-  // Allocate userdata
+
+  // Grab the table of all entries onto the stack
+  lua_rawgeti(L, LUA_REGISTRYINDEX, cronent_table_ref);
+
+  // Find a free index
+  int ix = 1;
+  while(lua_rawgeti(L, -1, ix) != LUA_TNIL) {
+    lua_pop(L, 1);
+    ix++;
+  }
+  lua_pop(L, 1); // pop the nil off the stack
+
+  // Allocate userdata onto the stack
   cronent_ud_t *ud = lua_newuserdata(L, sizeof(cronent_ud_t));
   // Set metatable
   luaL_getmetatable(L, "cron.entry");
@@ -100,29 +107,30 @@ static int lcron_create(lua_State *L) {
   lua_pushvalue(L, 2);
   ud->cb_ref = luaL_ref(L, LUA_REGISTRYINDEX);
   // Set entry
-  ud->desc = desc;
-  // Store entry
-  newlist = os_realloc(cronent_list, sizeof(int) * (cronent_count + 1));
-  if (newlist == NULL) {
-    return luaL_error(L, "out of memory");
-  }
-  lua_pushvalue(L, -1);
-  cronent_list = newlist;
-  cronent_list[cronent_count++] = luaL_ref(L, LUA_REGISTRYINDEX);
-  return 1;
+  lcron_parsedesc(L, strdesc, &ud->desc);
+
+  // Store entry to table while preserving userdata
+  lua_pushvalue(L, -1); // clone userdata
+  lua_rawseti(L, -3, ix); // -userdata; userdata, cronent table, cb, desc
+
+  return 1; // just the userdata
 }
 
+// Finds index, leaves table at top of stack for convenience
 static size_t lcron_findindex(lua_State *L, cronent_ud_t *ud) {
-  cronent_ud_t *eud;
-  size_t i;
-  for (i = 0; i < cronent_count; i++) {
-    lua_rawgeti(L, LUA_REGISTRYINDEX, cronent_list[i]);
-    eud = lua_touserdata(L, -1);
+  lua_rawgeti(L, LUA_REGISTRYINDEX, cronent_table_ref);
+  size_t count = lua_objlen(L, -1);
+
+  for (size_t i = 1; i <= count; i++) {
+    lua_rawgeti(L, -1, i);
+    cronent_ud_t *eud = lua_touserdata(L, -1);
     lua_pop(L, 1);
-    if (eud == ud) break;
-  }
-  if (i == cronent_count) return -1;
+    if (eud == ud) {
       return i;
+    }
+  }
+
+  return count + 1;
 }
 
 static int lcron_schedule(lua_State *L) {
@@ -131,17 +139,12 @@ static int lcron_schedule(lua_State *L) {
   struct cronent_desc desc;
   lcron_parsedesc(L, strdesc, &desc);
   ud->desc = desc;
+
   size_t i = lcron_findindex(L, ud);
-  if (i == -1) {
-    void *newlist;
-    newlist = os_realloc(cronent_list, sizeof(int) * (cronent_count + 1));
-    if (newlist == NULL) {
-      return luaL_error(L, "out of memory");
-    }
-    cronent_list = newlist;
-    lua_pushvalue(L, 1);
-    cronent_list[cronent_count++] = luaL_ref(L, LUA_REGISTRYINDEX);
-  }
+
+  lua_pushvalue(L, 1); // copy ud to top of stack
+  lua_rawseti(L, -2, i); // install into table
+
   return 0;
 }
 
@@ -157,27 +160,25 @@ static int lcron_handler(lua_State *L) {
 static int lcron_unschedule(lua_State *L) {
   cronent_ud_t *ud = luaL_checkudata(L, 1, "cron.entry");
   size_t i = lcron_findindex(L, ud);
-  if (i == -1) return 0;
-  luaL_unref(L, LUA_REGISTRYINDEX, cronent_list[i]);
-  memmove(cronent_list + i, cronent_list + i + 1, sizeof(int) * (cronent_count - i - 1));
-  cronent_count--;
+
+  lua_pushnil(L);
+  lua_rawseti(L, -2, i);
+
   return 0;
 }
 
+// scheduled entries are pinned, so we cannot arrive at the __gc metamethod
 static int lcron_delete(lua_State *L) {
   cronent_ud_t *ud = luaL_checkudata(L, 1, "cron.entry");
-  lcron_unschedule(L);
   luaL_unref(L, LUA_REGISTRYINDEX, ud->cb_ref);
   return 0;
 }
 
 static int lcron_reset(lua_State *L) {
-  for (size_t i = 0; i < cronent_count; i++) {
-    luaL_unref(L, LUA_REGISTRYINDEX, cronent_list[i]);
-  }
-  cronent_count = 0;
-  free(cronent_list);
-  cronent_list = 0;
+  lua_newtable(L);
+  luaL_unref(L, LUA_REGISTRYINDEX, cronent_table_ref);
+  cronent_table_ref = luaL_ref(L, LUA_REGISTRYINDEX);
+
   return 0;
 }
 
@@ -189,19 +190,27 @@ static void cron_handle_time(uint8_t mon, uint8_t dom, uint8_t dow, uint8_t hour
   desc.dow  = ( uint8_t)1 << dow;
   desc.hour = (uint32_t)1 << hour;
   desc.min  = (uint64_t)1 << min;
-  for (size_t i = 0; i < cronent_count; i++) {
-    lua_rawgeti(L, LUA_REGISTRYINDEX, cronent_list[i]);
+
+  lua_rawgeti(L, LUA_REGISTRYINDEX, cronent_table_ref);
+  size_t count = lua_objlen(L, -1);
+
+  for (size_t i = 1; i <= count; i++) {
+    lua_rawgeti(L, -1, i);
     cronent_ud_t *ent = lua_touserdata(L, -1);
     lua_pop(L, 1);
+
     if ((ent->desc.mon  & desc.mon ) == 0) continue;
     if ((ent->desc.dom  & desc.dom ) == 0) continue;
     if ((ent->desc.dow  & desc.dow ) == 0) continue;
     if ((ent->desc.hour & desc.hour) == 0) continue;
     if ((ent->desc.min  & desc.min ) == 0) continue;
+
     lua_rawgeti(L, LUA_REGISTRYINDEX, ent->cb_ref);
-    lua_rawgeti(L, LUA_REGISTRYINDEX, cronent_list[i]);
+    lua_rawgeti(L, -2, i); // get ud again
     luaL_pcallx(L, 1, 0);
   }
+
+  lua_pop(L, 1); // pop table
 }
 
 static void cron_handle_tmr() {
@@ -250,6 +259,10 @@ int luaopen_cron( lua_State *L ) {
     //My guess: To be sure to give the other modules required by cron enough time to get to a ready state, restart cron_timer.
   os_timer_arm(&cron_timer, 1000, 0);
   luaL_rometatable(L, "cron.entry", LROT_TABLEREF(cronent));
+
+  cronent_table_ref = LUA_NOREF;
+  lcron_reset(L);
+
   return 0;
 }